PROJECT SUMMARY The major goal of this proposal is to develop and validate therapeutic strategies targeting apolipoprotein E (APOE) gene with a specific focus on reducing the pathogenic effects of APOE4, the strongest genetic risk factor for late-onset Alzheimer's disease (AD). In our previous funding cycle, we have developed cell type- specific and inducible mouse models expressing human APOE3 or APOE4. Using these models, we have shown that expression of APOE4 in the brain or periphery leads to impairments in synaptic functions and enhancement of amyloid pathologies, whereas expression of APOE3 is either neutral or beneficial. The effects of peripherally expressed apoE4 on brain cognition and AD-related pathologies are particular exciting as we for the first time were able to separate the effects of brain and peripheral apoE4; the latter appears to modulate brain functions at least partially by compromising vascular integrity and function. Those findings also provide strong scientific premise that shifting apoE isoform status from the harmful apoE4 to neutral/beneficial apoE3 (or perhaps the protective apoE2) only in the periphery can reduce the risk and/or slow the progression of AD for APOE4 carriers in particular the APOE4 homozygotes. For this renewal application, we plan to perform preclinical testing using genetic or plasma exchange approaches. Our central hypothesis is that shifting apoE isoforms in the periphery from apoE4 to apoE3 can rescue AD-related pathology and cognitive decline by improving vascular function and promoting protective immune responses. We plan to test our hypothesis through three specific aims: In Aim 1, we plan to examine how peripheral APOE3 expression by the liver in the background of APOE4 genotype at different ages and different stages of the amyloid pathology impacts AD- related pathways. Peripheral apoE3 expression will be achieved by breeding our conditional APOE3 mouse model with Albumin-Cre (Alb-Cre) mice in the background of APOE4-targeted replacement (TR) mice. Effects on synaptic functions, behaviors, and AD-related pathology will be examined at different ages and different stages of amyloid pathology in both male and female mice. Single cell RNA sequencing studies will be carried out to uncover new pathways and networks impacted by APOE genotype changes. In Aim 2, we plan to address how macrophage APOE3 expression driven by LysM-Cre in the background of APOE4-TR impacts AD-related pathologies and pathways. In Aim 3, we will test whether blood exchange from apoE4 to apoE3 in APOE4-TR mice through parabiosis or infusion of plasma from young APOE3-TR mice can rescue AD-related pathologies and pathways. Successful completion of these studies should provide strong preclinical rationale and guidance for designing human clinical trials by modulating peripheral apoE isoforms, offering novel, individualized therapeutic strategies based on APOE genotype.